Method of extending the lifetime of thermal printing elements



J. O. PERCIVAL METHOD OF EXTENDING THE LIFETIME Filed Feb. 24, 1969 SELECTIONJ CIRCUITRY INVENTOR JOHN 0. PERCIV 1. MW

HIS ATTORNEYS United States Patent Oflice Patented Nov. 3, 1970 US. Cl. 219-216 1 Claim ABSTRACT OF THE DISCLOSURE A method of extending the lifetime of thermal printing elements having a negative temperature coefficient of resistivity by employing a driving voltage pulse of a particular waveshape is disclosed.

BACKGROUND OF THE INVENTION Thermal printing elements have been constructed by depositing electrically resistive stripes onto an insulated substrate so that these stripes present thermal printing areas. Tantalum is a material which has been deposited by conventional vacuum sputtering techniques to form these electrically resistive thermal printing elements; however, if vacuum sputtered tantalum is heated by the passage of electrical current, it exhibits a negative temperature coefiicient of resistivity, and, therefore, as the temperature of the tantalum printing element increases due to power dissipation in the printing element, the resistance of the printing element decreases. If a constant magnitude driving voltage pulse is repeatedly supplied across the printing element, a decreased lifetime of the printing element will result.

BRIEF DESCRIPTION OF THE DRAWING The figure shows a print head and a circuit which implement the method of the present invention.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT The thermal print head shown in the figure is constructed by the vacuum sputtering of tantalum in narrow stripes to form the printing elements 14 upon the glass substrate 12, and a conventional selection circuit 16 is employed to determine which of the printing elements are to be energized, so that the printing may occur in a selective manner on a thermally sensitive record material (not shown), which is placed adjacent to the thermal print head 10.

The circuit shown in the figure implements the method of the present invention. The NPN transistor 22 has its collector 28 coupled to the terminal 30, which is supplied by a positive voltage supply, and its emitter 24 connected to a resistor 26, which is coupled to the input terminal 18 of the selection circuitry 16. The selection circuitry 16 is supplied the driving voltage pulse for the selected thermal printing elements 14 through the input terminal 18. The base 32 of the transistor 22 is connected to the biasing resistor 34, which is connected between the base 32 of the NPN transistor 22 and earth, and the input resistor 46 and the input capacitor 50 are coupled in parallel between the base 32 of the transistor 22 and the input terminal 48.

When a constant magnitude voltage pulse is applied to the input terminal 48, the transistor 22 is driven into conduction, and a current will flow through the collectoremitter path of the transistor 22, through the resistor 26, and through the selected thermal printing elements 14 to earth. The driving voltage pulse for the thermal printing elements 14 that is developed by the circuit shown in the figure is shown at the emitter 24 of the transistor 22. This driving voltage pulse has a magnitude which initially is sufficient to raise the temperature of the selected printing elements 14 to the desired printing temperature, due to an initial surge of current that is supplied to the base 32 of the transistor 22 by the input capacitor 50, and which then decreases in magnitude in a predetermined manner as a function of time, so as to maintain the temperature of the selected printing elements 14 at a substantially constant value during printing.

What is claimed is:

1. A method of extending the lifetime of thermal printing elements that exhibit a negative temperature coefficient of resistivity upon the passage of electrical current therethrough, which comprises applying a voltage pulse to selected thermal printing elements that has a magnitude which initially is sufficient to raise the temperature of the selected printing elements to the desired printing temperature and which then decreases in magnitude in a predetermined manner as a function of time so as to maintain the temperature of the selected thermal printing elements at a substantially constant value during printing.

References Cited FOREIGN PATENTS 3/1962 Great Britain.

OTHER REFERENCES IBM Technical Disclosure Bulletin, vol. 7, No. 12, May 12, 1965, pp. 1152, 1153.

JOSEPH V. TRUH'E, Primary Examiner P. W. GOWDEY, Assistant Examiner US. Cl. X.R. 323-227, 25 

